Scientists have long been baffled as to how a particular sea slug is able to photosynthesize after ingesting algae. Now, a team of researchers appears to have found the answer: the slug steals genes from the algae that it eats.

The slug's special ability to steal genes from algae allows it to live like a plant for a long period of time and get nourishment from the sun. It also provides the first known example of horizontal gene transfer in multicellular organisms.

For the new study published in The Biological Bulletin on Dec. 1, Sidney Pierce from the Department of Integrative Biology at the University of South Florida, together with colleagues, used advanced imaging technology to spot a gene on the chromosome of the emerald sea slug (Elysia chlorotica), which came from an algae known as Vaucheria litorea.

The particular gene the researchers are interested in is crucial to the slug's ability to sustain the photosynthetic process that provides it with the food that it needs. It also produces an enzyme that plays a key role in the function of chloroplasts that are typically present in plants and algae.

Once the chloroplasts are inside the slug cells, they continue to photosynthesize for as long as nine months, a period far longer than how they perform in the algae. The photosynthesis process then produces the lipids and carbohydrates needed by the slug for nourishment.

"Here, we have used fluorescent in situ hybridization to localize an algal nuclear gene, prk, found in both larval and adult slug DNA by PCR and in adult RNA by transcriptome sequencing and RT-PCR," Pierce and colleagues reported. "The prk probe hybridized with a metaphase chromosome in slug larvae, confirming gene transfer between alga and slug."

The slug stealing chloroplasts from from V. litorea and embedding these into its own digestive system is a phenomenon known as kleptoplasty. Scientists have been interested in knowing how the slug is able to maintain these organelles for a long time.

Pierce explained that the gene is incorporated into the chromosome of the slug and transmitted to the next generation, and while these next-gen slugs would have to take up chloroplasts anew from the V. litorea, the genes that are needed to maintain the chloroplasts are already present on the genome of the animal.

"This paper confirms that one of several algal genes needed to repair damage to chloroplasts, and keep them functioning, is present on the slug chromosome," Pierce said. "The gene is incorporated into the slug chromosome and transmitted to the next generation of slugs."

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